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Abstract:

A frequency offset estimation unit estimates a frequency offset by
combining information on a maximum window having a maximum peak power
obtained from a received PRACH (Physical Random Access Channel) signal
and a sign of a phase of a correlation value between channel estimation
values obtained from a received PUSCH (Physical Uplink Shared Channel)
signal.

Claims:

1. A communication apparatus comprising: a channel estimation unit that
performs channel estimation from a received PUSCH (Physical Uplink Shared
Channel) signal; and a frequency offset estimation unit that estimates a
frequency offset by combining: information on a maximum window having a
maximum peak power, obtained from a received PRACH (Physical Random
Access Channel signal; and a sign of a phase of a correlation value
between channel estimation values obtained from the received PUSCH
signal.

2. The communication apparatus according to claim 1, wherein the
frequency offset estimation unit estimates the frequency offset from a
phase of a correlation value between the channel estimation values, based
on: a correspondence relationship between each frequency segment of a
first frequency segment group obtained by dividing an estimable frequency
offset range and the maximum window; and a relationship between each
frequency segment of a second frequency segment group obtained by
dividing the estimable frequency offset range and the sign of the
correlation value between the channel estimation values.

3. The communication apparatus according to claim 1, comprising: a PRACH
reception processing unit; and a PUSCH reception processing unit, wherein
the PRACH reception processing unit comprises at least: a plurality of
maximum path detectors that respectively detect maximum path powers from
the received PRACH signal, using a plurality of mutually different
windows; and a maximum path window selection unit that selects one of the
windows corresponding to the maximum power from among the maximum path
powers respectively detected by the maximum path detectors, and wherein
the PUSCH reception processing unit comprises at least: the channel
estimation unit performing channel estimation from a received reference
signal; the frequency offset estimation unit; and a demodulation unit
that performs frequency offset compensation using a frequency offset
amount estimated by the frequency offset estimation unit; the frequency
offset estimation unit estimating the frequency offset, using information
on the phase of the correlation value between the channel estimation
values and the sign of the phase from the channel estimation unit and the
information on the maximum window from the maximum path window selection
unit.

4. The communication apparatus according to claim 3, wherein the windows
comprise: a center window, a left window, and a right window, wherein
with argR [radian] being an argument of a complex correlation value R
between the channel estimation values obtained from the received PUSCH
signal and Ts[s] being a duration between reference symbols in a
former-half slot and a latter-half slot of a PUSCH subframe, the
frequency offset estimation unit determines a frequency offset Δf
by the following expression, in case the argR is greater than or equal to
0 and less than π, and the maximum window is the center window or the
right window: Δ f = arg R [ radian ] 2
π × 1 T S [ s ] ##EQU00023## in case the argR is
greater than or equal 0 to and less than π, and the maximum window is
the left window: Δ f = ( arg R [ radian ]
2 π - 1 ) × 1 T S [ s ] ##EQU00024## in
case the argR is greater than or equal to -.pi. and less than 0, and the
maximum window is the center window or the left window: Δ
f = arg R [ radian ] 2 π × 1 T S [
s ] ##EQU00025## and, in case the argR is greater than or equal to
-.pi. and less than 0 and the maximum window is the right window:
Δ f = ( arg R [ radian ] 2 π + 1
) × 1 T S [ s ] ##EQU00026## the estimable frequency
offset range being set to a range greater than or equal to -1/Ts
[Hz] and less than +1/Ts [Hz].

5. The communication apparatus according to claim 4, wherein with a path
search width being indicated by Nsearch, a preamble sequence length
being indicated by Nzc, and a distance between peaks of the center
window and each of the left and right windows being indicated by d, first
to third maximum path detectors respectively corresponding to the center
window, the left window, and the right window, determine maximum values
of squares of powers p (k), using a center window Wcenter={0, 1, . .
. , Nsearch-1}, a right window Wright={d-1, (d-1+1)
mod NZC, . . . , (d-1+Nsearch-1) mod NZC}, and a left
window Wleft={NZC-d-1, (NZC-d-1+1) mod NZC,
. . . , (NZC-d-1+Nsearch-1) mod NZC}, respectively.

6. The communication apparatus according to claim 3, wherein the maximum
path window selection unit selects the maximum window from among the
maximum path powers of the windows, based on the power adjusted by a
predetermined scaling value.

7. The A radio communication system comprising the communication
apparatus as set forth in claim 1.

8. The radio communication system comprising a base station that includes
the communication apparatus as set forth in claim 1.

9. A frequency offset estimation method, comprising: performing channel
estimation from a received PUSCH (Physical Uplink Shared Channel) signal;
and estimating a frequency offset by combining: information on a maximum
window having a maximum peak power, obtained from a received PRACH
(Physical Random Access Channel signal, and a sign of a phase of a
correlation value between channel estimation values obtained from the
received PUSCH signal.

10. The frequency offset estimation method according to claim 9,
comprising estimating the frequency offset from the phase of the
correlation value between the channel estimation values, based on: a
correspondence relationship between each frequency segment of a first
frequency segment group obtained by dividing an estimable frequency
offset range and the maximum window; and a relationship between each
frequency segment of a second frequency segment group obtained by
dividing the estimable frequency offset range and the sign of the
correlation value between the channel estimation values.

11. The frequency offset estimation method according to claim 9,
comprising: detecting respective maximum path powers of a plurality of
mutually different windows from the received PRACH signal, using the
plurality of mutually different windows; selecting one of the windows
corresponding to the maximum power from among the detected maximum path
powers; and estimating the frequency offset, using information on the
phase of the correlation value between the channel estimation values and
the sign of the phase and the information on the maximum window.

12. The frequency offset estimation method according to claim 11, wherein
the windows includes a center window, a left window, and a right window,
the method comprising: with argR [radian] being an argument of a complex
correlation value R between the channel estimation values obtained from
the received PUSCH signal being indicated and a duration between
reference symbols in a former-half slot, and Ts[s] being a
latter-half slot of a PUSCH subframe, determining a frequency offset
Δf according to the following expression, in case the argR is
greater than or equal to 0 and less than π, and the maximum window is
the center window or the right window: Δ f = arg
R [ radian ] 2 π × 1 T S [ s ]
##EQU00027## in case the argR is greater than or equal to 0 and less
than π, and the maximum window is the left window: Δ f
= ( arg R [ radian ] 2 π - 1 ) × 1
T S [ s ] ##EQU00028## in case the argR is greater than or equal
to -.pi. and less than 0, and the maximum window is the center window or
the left window: Δ f = arg R [ radian ]
2 π × 1 T S [ s ] ##EQU00029## and, in case the
argR is greater than or equal to -.pi. and less than 0, and the maximum
window is the right window: Δ f = ( arg R
[ radian ] 2 π + 1 ) × 1 T S [ s ]
##EQU00030## the estimable frequency offset range being set to a range
greater than or equal to -1/Ts [Hz] and less than +1/Ts [Hz].

13. The frequency offset estimation method according to claim 12, wherein
with a path search width being indicated by Nsearch, a preamble
sequence length being indicated by Nzc, and a distance between peaks
of the center window and each of the left and right windows being
indicated by d, first to third maximum path detectors respectively
corresponding to the center window, the left window, and the right
window, respectively determining maximums values of squares of powers p
(k), using a center window Wcenter={0, 1, . . . , Nsearch-1}, a
right window Wright={d-1(d-1+1) mod NZC, . . . ,
(d-1+Nsearch-1) mod NZC}, and a left window
Wleft={NZC-d-1, (NZC-d-1+1) mod NZC, . . .
, (NZC-d-1+Nsearch-1) mod NZC}, respectively.

14. The frequency offset estimation method according to claim 11,
comprising selecting the maximum window from among the maximum path
powers of the windows, based on the power adjusted by a predetermined
scaling value.

16. A computer-readable storage medium having a frequency offset
estimation program stored therein for causing a computer to execute the
processing comprising: performing channel estimation from a received
PUSCH (Physical Uplink Shared Channel) signal; and estimating a frequency
offset by combining: information on a maximum window having a maximum
peak power, obtained from a received PRACH (Physical Random Access
Channel signal; and a sign of a phase of a correlation value between
channel estimation values obtained from a received PUSCH signal.

17. The storage medium according to claim 16, wherein the frequency
offset estimation processing estimates the frequency offset from the
phase of the correlation value between the channel estimation values,
based on: a correspondence relationship between each frequency segment of
a first frequency segment group obtained by dividing an estimable
frequency offset range and the maximum window; and a relationship between
each frequency segment of a second frequency segment group obtained by
dividing the estimable frequency offset range and the sign of the
correlation value between the channel estimation values.

18. The storage medium according to claim 16, wherein the program stored
therein causes the computer to execute the processing comprising:
detecting respective maximum path powers of a plurality of mutually
different windows from the received PRACH signal, using the plurality of
mutually different windows; selecting a window corresponding to the
maximum power from among the detected maximum path powers; and estimating
the frequency offset, using information on the phase of the correlation
value between the channel estimation values and the sign of the phase and
the information on the maximum window from the maximum path window
selection unit.

19. The storage medium according to claim 18, wherein the windows
includes a center window, a left window, and a right window, the program
stored in the storage medium causing the computer to execute the
processing comprising: with argR [radian] being an argument of a complex
correlation value R between the channel estimation values obtained from
the received PUSCH signal, and Ts[s] being a duration between
reference symbols in a former-half slot and a latter-half slot of a PUSCH
subframe, determining a frequency offset Δf according to the
following expression, in case the argR is greater than or equal to 0 and
less than π, and the maximum window is the center window or the right
window: Δ f = arg R [ radian ] 2 π
× 1 T S [ s ] ##EQU00031## in case the argR is
greater than or equal to 0 and less than π, and the maximum window is
the left window: Δ f = ( arg R [ radian ]
2 π - 1 ) × 1 T S [ s ] ##EQU00032## in
case the argR is greater than or equal to -.pi. and less than 0, and the
maximum window is the center window or the left window: Δ
f = arg R [ radian ] 2 π × 1 T S [
s ] ##EQU00033## and, in case the argR is greater than or equal to
-.pi. and less than 0, and the maximum window is the right window:
Δ f = ( arg R [ radian ] 2 π + 1
) × 1 T S [ s ] ##EQU00034## the estimable frequency
offset range being set to a range greater than or equal to -1/Ts
[Hz] and less than +1/Ts [Hz].

20. The storage medium according to claim 18, wherein the program stored
therein causes the computer to execute the processing comprising
selecting the maximum window from among the maximum path powers of the
windows, based on the power adjusted by a predetermined scaling value.

Description:

TECHNICAL FIELD

Description of Related Application

[0001] The present invention is based upon and claims the benefit of the
priority of Japanese Patent Application No. 2009-260457 (filed on Nov.
13, 2009), the disclosure of which is incorporated herein in its entirety
by reference.

[0002] The present invention relates to a radio communication system. More
specifically, the invention relates to a communication apparatus for
estimating and compensating for a frequency offset, a frequency offset
estimation method, a radio communication system, and a program.

BACKGROUND

[0003] In 3GPP (3rd Generation Partnership Project) LTE (Long Term
Evolution), DFT (Discrete Fourier Transform)-spread-OFDM (Orthogonal
Frequency Division Multiplexing) is adopted as an uplink access scheme.
Generally, in the DFT-spread-OFDM, due to movement of a terminal, a
frequency deviation of an oscillator, or the like, a carrier frequency
difference (hereinafter referred as a "frequency offset") occurs between
a receiver and a transmitter. When the frequency offset occurs, a
transmission characteristic of a signal deteriorates. In order to prevent
the frequency offset, estimation of a frequency offset amount and
frequency offset compensation are generally performed on a receiver side.

[0004] The following describes

[0005] Initial Access Procedure in the 3GPP LTE; and

[0006] General Transmission and Reception Processing; and then

[0007] Frequency Offset Estimation and Compensation and Problem of the
Frequency Offset Estimation and Compensation

[0008] First, an overview of the initial access procedure will be
described. In the LTE, a random access procedure is used as means for
establishing synchronization of an uplink when a terminal undergoes a
transition from an idle state to a connection state. FIG. 1 is a diagram
illustrating the random access procedure. FIG. 1 corresponds to the
figure (FIG. 10.1.5. 1-1 Contention Based Random Access Procedure) on
page 53 in Non-patent Document 1.

[0009] As shown in FIG. 1, the random access procedure is constituted from
the following four steps:

[0012] In the LTE, a frequency and time resource for the random access
preamble in FIG. 2 is provided in advance for step 1 in FIG. 1. The
terminal transmits the random access preamble in step 1, using the
frequency and time resource specified for random access when the terminal
undergoes a transition from the idle state to the connection state.

[0014] In the reception processing unit in FIG. 4, cyclic prefixes are
removed from a received signal received, using the frequency and time
resource for the random access, by the cyclic prefix removal unit 11.

[0015] Next, the DFT unit 12 performs DFT on a signal having the cyclic
prefixes removed. The subcarrier demapping unit 13 performs subcarrier
demapping on a signal after the DFT (in a frequency domain) to extract a
signal corresponding to a frequency resource specified for the random
access.

[0016] The preamble signal multiplication unit 14 multiplies a signal
subjected to subcarrier demapping by the subcarrier demapping unit 13
with a complex conjugate of a transmission preamble signal.

[0017] The IDFT unit 15 performs IDFT on the output signal of the preamble
signal multiplication unit 14. The maximum path detection unit 16
calculates power per sample of the signal after the IDFT (in a time
domain). The calculated power per sample is referred to as a "PRACH
correlation value".

[0018] The maximum path detection unit 16 determines peak (maximum) power
of the PRACH correlation value. If the peak (maximum) power is equal to
or larger than a preset threshold, the base station regards that the
terminal has transmitted the preamble. The base station transmits the
random access response in FIG. 1 by a downlink.

[0020] FIG. 3 illustrates a PUSCH subframe (Subframe) format. Duration of
one subframe is 1 ms. The one subframe includes a 14 number of
DFT-Spread-OFDM symbols and a CP (Cyclic Prefix: cyclic prefix)
associated with each of the 14 number of DFT-Spread-OFDM symbols.

[0021] Third and tenth symbols from a 0th symbol (symbol on the left end
of the 14 symbols) are referred to as reference symbols (Reference
Symbols) (indicated by RSs). A known sequence is transmitted to and from
both of a receiving side and a transmitting side as the reference
symbols, and is used for channel estimation and frequency offset
estimation for demodulating data on the receiving side. 12 symbols of the
0th, first, second, fourth, fifth, sixth, seventh, eighth, ninth,
eleventh, twelfth, and thirteenth symbols (each indicated by D) other
than the reference symbols are used for data transmission. The duration
of a former-half slot (Slot #0) of the subframe is set to be 0.5 ms, and
the duration of a latter-half slot (Slot #1) is set to be 0.5 ms. The
duration between the reference symbols (RSs) in the former-half and the
latter-half slots is set to be 0.5 ms.

[0022] On receipt of an uplink signal by the base station in step 3 in
FIG. 1, the base station executes downlink data transmission in response
to the uplink signal, in step 4 in FIG. 1.

[0023] Next, general PUSCH reception processing in step 3 in FIG. 1 will
be described. FIG. 5 illustrates, as a prototype example (reference
case), a general configuration of a PUSCH reception processing unit of
the receiver on the side of the base station. The reception processing
unit includes a cyclic prefix removal unit 21, DFT units 22-1 and 22-2,
subcarrier demapping units 23-1 and 23-2, a reference signal
multiplication unit 24, a channel estimation unit 25, a frequency offset
estimation unit 26, a data equalization unit 27, and a demodulation unit
28.

[0024] The cyclic prefix removal unit 21 removes cyclic prefixes from a
received PUSCH signal to divide the resulting signal into a data signal
and reference symbols.

[0025] The DFT units 22-1 and 22-2 perform DFT on the data signal and the
reference symbols received and obtained by the division. The subcarrier
demapping units 23-1 and 23-2 performs subcarrier demapping on the signal
after the DFT (in the frequency domain) to extract a frequency domain
signal allocated to the user.

[0028] Next, the obtained channel estimation value and a data signal after
the subcarrier demapping are supplied to the data equalization unit 27.
The data equalization unit 27 equalizes the frequency domain of the data
signal.

[0029] Finally, the demodulation unit 28 converts the signal equalized in
the frequency domain by the data equalization unit 27 into a time domain
signal. Further, the demodulation unit 28 performs offset compensation on
the signal converted into the time domain signal, using the frequency
offset amount estimated by the frequency offset estimation unit 26.

[0030] A general example of the frequency offset compensation performed on
the signal converted into the time domain signal is given by the
following expression (1):

sdem,comp(k)=sdem(k)exp(-j2πkΔfΔT); k=0, 1, 2,
(1)

[0031] where Sdem(k) (k=0, 1, 2, . . . ) is a complex signal (before
the frequency offset compensation) converted into the time domain signal
by the demodulation unit 28.

[0032] Sdem,comp(k) (k=0, 1, 2, . . . ) is a signal (after the
frequency offset compensation) obtained by the frequency offset
compensation performed on the complex signal converted into the time
domain signal by the demodulation unit 28.

[0034] ΔT[s] is the duration of one sample of the demodulated signal
converted into the time domain signal by the demodulation unit 28.

[0035] The frequency offset amount during initial access is obtained by
taking correlation between complex channel estimation values of two slots
obtained from the reference symbols (RSs) of the former-half and
latter-half slots of the PUSCH signal, and by further obtaining an
argument of the complex correlation value. This process will be described
below in detail.

[0036] First, it is assumed that a complex channel estimation value as
follows is obtained for each subcarrier allocated to the user:

H(s,k) (2)

[0037] where s is a slot number in one subframe, and s=0, 1 (0:
former-half slot, 1: latter-half slot) (refer to FIG. 3).

[0038] k is a sub carrier number, and k=0, 1, . . . , N-1 (where N
indicates the number of subcarriers allocated to the user).

[0039] The frequency offset amount Δf[Hz] is estimated as shown in
the following Expressions (3) and (4), using the channel estimation
values of two slots.

[0040] A correlation value R between the channel estimation values (H(s=0,
k) and H(s=1, k)) of the two slots obtained from the received PUSH signal
is calculated according to the following Expression (3):

[0044] where Ts[s] indicates the duration [unit: s (second)] between
the two reference symbols (RSs) in the former-half and latter-half slots
of the PUSCH signal, and

[0045] argR indicates an argument [unit: radian (radian)] of the complex
correlation value R between the channel estimation values, and ranges
from -π to +π.

[0046] Estimation of the frequency offset amount Δf, using the
Expression (4) utilizes the fact that the argument argR of the complex
correlation value R between the channel estimation values obtained from
the PUSCH signal is given by:

2πΔfTs[radian] (5)

[0047] In the LTE, it is defined that Ts=0.5 [ms]
(=0.5×10-3) (refer to FIG. 3). Thus, the above Expression (4)
becomes as shown in the following Expression (6):

[0048] In case the abovementioned estimation method is used, an estimable
frequency offset range [Hz] is uniquely determined by the following
expression (7), based on the duration Ts [second] between the reference
symbols (RSs) of the two slots (former-half and latter-half slots) to be
correlated. The relational expression (7) is derived from the expression
(4) of Δf=(argR)/(2πTs), where argR is in a relationship of
-π≦argR<.

[0049] In the PUSCH subframe format specifications of the LTE (where
Ts=0.5 [ms] (=0.5×10-3)), the estimable frequency offset
range becomes from -1000 Hz to 1000 Hz based on the expression (7).

[0050] Patent Document 1 describes configurations of a control signal and
a data signal (including a preamble transferred on a PRACH and a
reference signal included in a frame on a PUSCH) on an air interface
between a base station apparatus and a terminal apparatus, the terminal
apparatus, and the base station apparatus in an LTE communication system.
Patent Document 2 discloses an apparatus for quickly and correctly
detecting a preamble code and a method (of estimating an integer carrier
frequency offset of a target base station by detecting a preamble index
of the target base station to select a target cell) in an environment
where there is a carrier frequency offset. Patent Document 3 discloses a
configuration or the like for determining a first frequency offset of a
received signal sequence, using first and second preambles. [0051][Patent Document 1][0052] JP Patent Kokai Publication No. JP2008-136172A
[0053][Patent Document 2][0054] JP Patent Kokai Publication No.
JP2008-236744A [0055][Patent Document 3][0056] JP Patent Kohyou
Publication No. JP2002-518880A [0057] [Non-patent Document 1] [0058] 3GPP
TS 36.300 V8.9.0 (2009-06) 101.5 Random Access Procedure [0059]
[Non-patent Document 2] [0060] 3GPP TS 36.211 V8.7.0 (2009-05) 5.7
Physical Random Access Channel [0061] [Non-patent Document 3] [0062] 3GPP
TS36.104 V9.0.0 (2009-06) B.3 High Speed Train Condition

SUMMARY

[0063] An analysis by the present invention will be given below. According
to Non-patent Document 3, it is necessary to compensate for a frequency
offset in the range of -1340 to +1340 Hz at a base station in the LTE.

[0064] On contrast therewith, the frequency offset estimable range in the
above-mentioned estimation of the frequency offset during the initial
access is from -1000 Hz to 1000 Hz. That is, there is a problem that the
frequency offset in the range of -1000 to -1340 Hz and the frequency
offset in the range of 1000 to 1340 Hz specified in Non-patent Document 3
cannot be correctly estimated.

[0065] When the frequency offset is +1100 Hz, for example, the phase argR
(-π≦argR<π) of the correlation value R between the two
reference symbols assumes 1.1 π (→-0.9 π) [radian], as given
by the following expression (8).

[0066] The frequency offset Δf is estimated to be -900 Hz as shown
by the following expression (9), using the expression (6).

[0067] That is, the frequency offset cannot be correctly estimated. When
the frequency offset is in the range of -1000 to -1340 Hz, there is the
same problem.

[0068] Accordingly, an object of the present invention is to provide a
method, an apparatus, a system, and a program whereby an estimable
frequency offset range can be expanded and a frequency offset can be
correctly estimated.

[0069] According to a first aspect of the present invention, there is
provided a communication apparatus comprising

[0070] a frequency offset estimation unit that estimates a frequency
offset by combining:

[0071] information on a maximum window having a maximum peak power,
obtained from a received PRACH (Physical Random Access Channel: physical
random access channel) signal; and

[0072] a sign of a phase of a correlation value between channel estimation
values obtained from a received PUSCH (Physical Uplink Shared Channel:
physical uplink shared channel) signal.

[0073] According to a second aspect of the present invention, there is
provided a frequency offset estimation method comprising estimating a
frequency offset by combining:

information on a maximum window having a maximum peak power, obtained
from a received PRACH (Physical Random Access Channel: physical random
access channel) signal; and a sign of a phase of a correlation value
between channel estimation values obtained from a received PUSCH
(Physical Uplink Shared Channel: physical uplink shared channel) signal.

[0074] According to a third aspect of the present invention, there is
provided a frequency offset estimation program for causing a computer
(processor) to execute the processing comprising:

estimating a frequency offset by combining:

[0075] information on a maximum window having a maximum peak power,
obtained from a received PRACH (Physical Random Access Channel: physical
random access channel) signal; and

[0076] a sign of a phase of a correlation value between channel estimation
values obtained from a received PUSCH (Physical Uplink Shared Channel:
physical uplink shared channel) signal. There is also provided a storage
medium (a memory device, a magnetic/optical disk medium/device) having
the frequency offset estimation program according to the present
invention stored therein.

[0077] According to the present invention, an estimable frequency offset
range can be expanded, and a frequency offset can be correctly estimated.

[0078] Still other features and advantages of the present invention will
become readily apparent to those skilled in this art from the following
detailed description in conjunction with the accompanying drawings
wherein only exemplary embodiments of the invention are shown and
described, simply by way of illustration of the best mode contemplated of
carrying out this invention. As will be realized, the invention is
capable of other and different embodiments, and its several details are
capable of modifications in various obvious respects, all without
departing from the invention. Accordingly, the drawing and description
are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] FIG. 1 is a diagram illustrating a random access procedure;

[0080] FIG. 2 is a diagram illustrating a PRACH preamble format;

[0081] FIG. 3 is a diagram illustrating a PUSCH subframe format;

[0082] FIG. 4 is a diagram illustrating a configuration of a PRACH
receiver;

[0083] FIG. 5 is a diagram illustrating a configuration of a PUSCH
receiver;

[0084] FIG. 6 is a graph illustrating a PRACH correlation value in case of
Δf=0 [Hz];

[0085] FIG. 7 is a graph illustrating a PRACH correlation value in case of
Δf=300 [Hz];

[0086] FIG. 8 is a graph illustrating a PRACH correlation value in case of
Δf=625 [Hz];

[0087] FIG. 9 is a graph illustrating a PRACH correlation value in case of
Δf=1100 [Hz];

[0088] FIG. 10 is a graph illustrating a PRACH correlation value in case
of Δf=-300 [Hz];

[0089] FIG. 11 is a graph illustrating a PRACH correlation value in case
of Δf=-625 [Hz];

[0090] FIG. 12 is a graph illustrating a PRACH correlation value in case
of Δf=-1100 [Hz];

[0091] FIG. 13 is a graph illustrating a relationship among peak powers of
center, left, and right windows.

[0092] FIG. 14 is a diagram illustrating a configuration of a single
carrier transmission system in an exemplary embodiment of the present
invention;

[0093] FIG. 15 is a diagram illustrating a configuration of a (PRACH)
transmitter in the single carrier transmission system in the exemplary
embodiment of the present invention;

[0094] FIG. 16 is a diagram illustrating a configuration of a (PUSCH)
transmitter in the single carrier transmission system in the exemplary
embodiment of the present invention;

[0095] FIG. 17 is a diagram illustrating a configuration of a (PRACH)
receiver in the single carrier transmission system in the exemplary
embodiment of the present invention;

[0096] FIG. 18 is a diagram illustrating a configuration of a (PUSCH)
receiver in the single carrier transmission system in the exemplary
embodiment of the present invention; and

[0097] FIG. 19 is a graph illustrating error detection probabilities.

PREFERRED MODES

[0098] Preferred modes of the present invention will be described.

[0099] One of the preferred modes of the present invention is given by the
above described first aspect.

[0100] According to a second mode of the present invention, the frequency
offset estimation unit estimates the frequency offset from the phase of
the correlation value between the channel estimation values, based on:

[0101] a correspondence relationship between each frequency segment of a
first frequency segment group obtained by dividing an estimable frequency
offset range and the maximum window; and

[0102] a relationship between each frequency segment of a second frequency
segment group obtained by dividing the estimable frequency offset range
and the sign of the correlation value between the channel estimation
values.

[0103] According to a third mode of the present invention, the
communication apparatus comprises:

[0106] a plurality of maximum path detectors that respectively detect
maximum path powers from the received PRACH signal, using a plurality of
mutually different windows; and

[0107] a maximum path window selection unit that selects one of the
windows corresponding to the maximum power from among the maximum path
powers respectively detected by the maximum path detectors. The PUSCH
reception processing unit comprises at least:

[0108] a channel estimation unit that performs channel estimation from a
received reference signal;

[0109] the frequency offset estimation unit; and

[0110] a demodulation unit that performs frequency offset compensation
using a frequency offset amount estimated by the frequency offset
estimation unit. The frequency offset estimation unit estimates the
frequency offset, using information on the phase of the correlation value
between the channel estimation values and the sign of the phase from the
channel estimation unit and the information on the maximum window from
the maximum path window selection unit.

[0111] According to a fourth mode of the present invention, the windows
comprise a center window, a left window, and a right window. Assuming
that argR [radian] is an argument of a complex correlation value R
between the channel estimation values obtained from the received PUSCH
signal and Ts[s] is a duration between reference symbols in a
former-half slot and a latter-half slot of a PUSCH subframe,

[0112] the frequency offset estimation unit determines a frequency offset
Δf according to the following expression,

in case argR is greater than or equal to 0 and less than π, and the
maximum window is the center window or the right window:

Δ f = arg R [ radian ] 2 π
× 1 T S [ s ] ##EQU00007##

in case argR is greater than or equal to 0 and less than π, and the
maximum window is the left window:

Δ f = ( arg R [ radian ] 2 π -
1 ) × 1 T S [ s ] ##EQU00008##

in case argR is greater than or equal to -π and less than 0, and the
maximum window is the center window or the left window:

Δ f = arg R [ radian ] 2 π
× 1 T S [ s ] ##EQU00009##

and in case argR is greater than or equal to -π and less than 0, and
the maximum window is the right window:

Δ f = ( arg R [ radian ] 2 π +
1 ) × 1 T S [ s ] ##EQU00010##

the estimable frequency offset range being set to a range between
-1/Ts [Hz] and less than +1/Ts [Hz].

[0113] According to a fifth mode of the present invention, in case

a path search width is indicated by Nsearch, a preamble sequence
length is indicated by Nz, and a distance between peaks of the
center window and each of the left and right windows is indicated by d,

[0114] first to third maximum path detectors respectively corresponding to
the center window, the left window, and the right window respectively
determine maximum values of squares of powers p (k), using

[0115] According to a sixth mode of the present invention, in the
selection of the maximum window, the maximum window is selected from
among the maximum path powers of the windows, based on the power adjusted
by a predetermined scaling value.

[0116] In a method according to the present invention, frequency offset
estimation is performed by combining:

[0117] a peak power of a correlation value obtained from a received PRACH
signal in step 1 of FIG. 1; and

[0118] a correlation value between channel estimation values obtained from
a received PUSCH (Physical Uplink Shared Channel: physical uplink shared
channel) signal in step 3 of FIG. 1. In the related art, frequency offset
estimation is performed using only the PUSCH received signal in step 3 of
FIG. 1.

[0119] As will be described below, according to the present invention, an
estimable range of the frequency offset Δf can be expanded from the
range of -1000 Hz to 1000 Hz of the related art to the range of -2000 Hz
to 2000 Hz.

[0121] In case there is no frequency offset (Δf=0 [Hz]), a sharp
peak occurs at k=0, in a center window (center_window), and no sharp peak
is present in each of left and right windows (righ_window, left_window)
with respect to the center window, as shown in FIG. 6.

[0122] On contrast therewith, in case there is a frequency offset, it can
be seen that there is a peak in each of the left and right windows as
well from FIGS. 7 and 10 (indicating the PRACH correlation values when
Δf=±300 [Hz]), FIGS. 8 and 11 (indicating the PRACH correlation
values when Δf=±625 [Hz]), FIGS. 9 and 12 (indicating the PRACH
correlation values when Δf=±1100 [Hz]).

[0123] Herein, it should be noted that a window including k=0 is defined
as the center window (center_window), a window including k=u-1 is
defined as the right window (right_window), and a window including
k=NZC-u-1 is defined as the left window (left_window). This is
a property specific to a Zadoff-Chu sequence, used for a PRACH preamble.

[0124] A distance (time difference) d between the peaks of the center
window and each of the left and right windows is uniquely determined by
the following expression (10) with respect to an arbitrary u.

d=u-1 mod NZC (10)

[0125] where u is a parameter "u" (Physical Root Sequence Index) in a
definition expression of xu(n) of a PRACH preamble in 5.7.2 Preamble
Sequence Generation on page 39 of Non-patent Document 2.

[0127] u-1 is a multiplicative inverse of u, and is a number equal to
or larger than 1 but smaller than Nzc, multiplication of which by u
results in 1 (uu-1=u-1u≡1 mod Nzc), under the
condition of mod Nzc.

[0129] That is, the presence of a frequency offset (Δf≠0) in
this example indicates that, in addition to the peak present at k=0 in
the center window (center_window), peaks also are present at k=u-1
(=280) in the right window (right_window) and at k=Nzc-u-1
(=559) in the left window (left_window), under the influence of the
frequency offset.

[0130] It can be seen from the respective PRACH correlation values, at
Δf=0 [Hz], Δf=±300 [Hz], Δf=±625 [Hz], and
Δf=±1100 [Hz] in FIGS. 6 to 12 that the magnitude of the peak of
the correlation value obtained from the received PRACH signal in each
window greatly depends on the frequency offset amount.

[0131] If there is no frequency offset, or in case of Δf=0 [Hz] in
FIG. 6, for example, there is a sharp peak present only at a point of
k=0. On contrast therewith, in cased of other frequency offsets, such as
in cases of Δf=±300 [Hz] in FIGS. 7 and 10, power peaks smaller
than the peak in the center window (center_window) are observed at
k=u-1 (k=280 in this example) in the right window (right_window) and
at k=839-u-1 (k=559 in this example) in the left window
(left_window).

[0132] In case of the frequency offset Δf=625 [Hz] in FIG. 8, the
peak power in the center window (center_window) matches the peak power in
the right window (right_window). In case of the frequency offset of
Δf=1100 [Hz] in FIG. 9, the peak power in the right window
(right_window) exceeds the peak power in the center window
(center_window).

[0133] In case of the frequency offset Δf=-625 [Hz] in FIG. 11, the
peak power in the center window (center_window) matches the peak power in
the left window (left_window). In case of the frequency offset
Δf=-1100 [Hz] in FIG. 12, the peak power in the left window
(left_window) exceeds the peak power in the center window
(center_window).

[0134] FIG. 13 is a graph illustrating a relationship between a frequency
offset and peak power in each of the center, right, and left windows. A
horizontal axis in FIG. 13 indicates the frequency offset (Frequency
Offset: Δf [Hz]), and a vertical axis indicates the peak power
[dB]. A triangle symbol indicates the peak power in the center window
(center_window), a rectangle symbol indicates the peak power in the right
window (right_window), and a symbol x indicates the peak power in the
left window (left_window).

[0135] It can be seen from FIG. 13 that the following relationships (A1)
to (A3) hold according to each frequency offset range. It should be noted
that these relationships do not depend on the parameter u (Physical Root
Sequence Index).

(A1) In the range of -2000 Hz≦frequency offset≦-625 Hz, the
following relationship holds,

[0136] (the peak power in the center window)<(the peak power in the
left window), and

[0137] (the peak power in the right window)<(the peak power in the left
window).

[0138] Accordingly, the peak power in the left window is maximum.

(A2) In the range of -625 Hz≦frequency offset≦625 Hz, the
following relationship holds,

[0139] (the peak power in the center window)>(the peak power in the
left window), and

[0140] (the peak power in the right window)<(the peak power in the
center window).

[0141] Accordingly, the peak power in the center window is maximum.

(A3) In the range of 625 Hz≦frequency offset≦2000 Hz, the
following relationship holds,

[0142] (the peak power in the center window)<(the peak power in the
right window), and

[0143] (the peak power in the left window)<(the peak power in the right
window).

[0144] Accordingly, the peak power in the right window is maximum.

[0145] The phase argR [radian] of the correlation value R between the
channel estimation values obtained from the received PUSCH signal is
calculated by the following expression (11):

2π×Δf[Hz]×Ts[s] (11)

[0146] For this reason, with the frequency offset (Frequency Offset:
Δf [Hz]) being in the range of from -2000 Hz to 2000 Hz, the
following relationships (B1) to (B3) hold with respect to a sign of the
phase (argR: -π˜π [radian]) of the correlation value R
between the PUSCH channel estimation values:

(B1) In the range of -2000 Hz≦the frequency offset≦-1000
Hz, the sign of the phase (argR) of the correlation value R between the
PUSCH channel estimation values is +. (B2) In the range of -1000
Hz≦the frequency offset≦0 Hz, the sign of the phase (argR)
of the correlation value R between the PUSCH channel estimation values is
-. (B3) In the range of 0 Hz≦the frequency offset≦1000 Hz,
the sign of the phase (argR) of the correlation value R between the PUSCH
channel estimation values is +. (B4) In the range from 1000 Hz≦the
frequency offset≦2000 Hz, the sign of the phase (argR) of the
correlation value R between the PUSCH channel estimation values is -.

[0147] By combining the two properties described in the relationships (A1)
to (A3) and the relationships (B1 to B4), which are:

[0148] information on the window (center/right/left window) having the
maximum peak of the correlation value obtained from the PRACH signal; and

[0149] the sign of the phase of the correlation value between the channel
estimation values obtained from the PUSCH signal,

[0150] to perform estimation of the frequency offset Δf [Hz], as
shown in the following expressions (12) to (15),

[0151] the estimable range of the frequency offset can be expanded from
the range of -1000 Hz to 1000 Hz to the range of -2000 Hz to 2000 Hz in
case of the LTE (Ts=0.5×10-3 [s]).

[0152] In the expressions (12) to (15), Δf [Hz] is a frequency
offset estimation value.

[0153] MaxWindow=right means that peak power in the right window is
maximum (maximum window=right window).

[0154] MaxWindow=center means that peak power in the center window is
maximum (maximum window=center window).

[0155] MaxWindow=left means that peak power in the left window is maximum
(maximum window=left window).

[0156] In case the frequency offset is +1100 Hz, for example, the phase
argR of the correlation value R between the reference symbols becomes
equal to -0.9 π [radian], as shown in the above expression (8). In
this case, the maximum window becomes the right window based on the
relationship (A3), and based on the expression (15), the frequency offset
estimation value Δf [Hz] is obtained:

[0158] In the estimable frequency offset range of
-1/TS≦Δf<1/TS in the above expression (18), a lower limit
-1/TS corresponds to the frequency offset estimation value Δf when
the arg R [radian]=-π in the above expression (13), and 1/TS
corresponds to the frequency offset estimation value Δf when the
phase argR [radian] is set to π in the above expression (15). The
following describes exemplary embodiments.

Exemplary Embodiments

[0159] FIG. 14 is a block diagram illustrating a configuration of a single
carrier transmission system according to an exemplary embodiment of the
present invention. The single carrier transmission system in this
exemplary embodiment is constituted from a transmitter 1 and a receiver
2.

[0160] FIG. 15 is a diagram illustrating a configuration of a PRACH
transmission processing unit of the transmitter in the single carrier
transmission system in FIG. 14. FIG. 16 is a diagram illustrating a
configuration of a PUSCH transmission processing unit of the transmitter
in the single carrier transmission system in FIG. 14.

[0162] Referring to FIG. 16, the PUSCH transmission processing unit
includes a PUSCH data signal generation unit 41, a PUSCH reference signal
generation unit 42, a DFT unit 43, a subcarrier mapping unit 44, an IDFT
unit 45, and a cyclic prefix addition unit 46. The block configuration
shown in each of FIGS. 14 and 15 shows an example of a configuration of a
common transmitter in the single carrier transmission system. The
transmission processing units are not always limited to such
configurations. Operation at a time of a PUSCH transmission is as
follows.

[0163] The PUSCH data signal generation unit 41 and the PUSCH reference
signal generation unit 42 generates transmission data signals and
transmission reference symbols respectively. Those signals are
time-division multiplexed according to a subframe format shown in FIG. 3,
and the DFT unit 43 performs DFT on the time-division multiplexed signal.
The subcarrier mapping unit 44 performs mapping of each frequency
component after the DFT onto a subcarrier allocated to each use. The IDFT
unit 45 performs IDFT on the signal after the mapping by the subcarrier
mapping unit 44. Finally, the cyclic prefix addition unit 46 adds cyclic
prefixes to a signal after the IDFT by, for transmission.

[0164] In case of a PRACH, the preamble signal generation unit 31
generates a preamble signal and the DFT unit 32 performs DFT on the
generated signal. Subsequent processing is the same as in case of the
PUSCH transmission.

[0165] Next, a configuration of a receiver in the single carrier
transmission system in the exemplary embodiment of the present invention
will be described. FIG. 17 is a diagram illustrating a configuration of a
receiver (PRACH reception processing unit) in the single carrier
transmission system in FIG. 15. FIG. 18 is a diagram illustrating a
configuration of a receiver (PUSCH reception processing unit) in the
single carrier transmission system in FIG. 15.

[0168] This exemplary embodiment is different from the configuration in
FIG. 5 in that maximum path window selection information from the PRACH
reception is added to data input to the frequency offset estimation unit
26A. FIGS. 17 and 18 illustrate examples of the exemplary embodiment of
the present invention. The present invention is not of course limited to
such configurations.

[0169] In case of the PRACH reception, the cyclic prefix removal unit 11
removes cyclic prefixes from a received PRACH signal. Then, the DFT unit
12 performs DFT on a signal having the cyclic prefixes removed. The
subcarrier demapping unit 13 performs subcarrier demapping on a signal
after the DFT to extract a signal corresponding to a frequency resource
specified for random access.

[0170] The preamble signal multiplication unit 14 multiplies a signal
after the subcarrier demapping with a complex conjugate of a transmission
preamble signal.

[0171] The IDFT unit 15 performs IDFT on data after the multiplication.
Then, the maximum path detection units 16R, 16C, and 16L detect maximum
paths for three (center, right, and left) windows respectively. The
maximum path window selection unit 17 in the subsequent stage selects one
of the windows having the maximum power from among maximum path powers of
the three windows (details of which will be described later).

[0172] In case of the PUSCH reception, the cyclic prefix removal unit 21
removes cyclic prefixes from a received PUSCH signal to divide the
resulting signal into a data signal and reference symbols.

[0173] Next, the DFT units 22-1 and 22-2 perform DFTs on the received data
signal and the received reference symbols, respectively. The subcarrier
demapping units 23-1 and 23-2 perform subcarrier demapping on the signal
after the DFT respectively to extract a signal in a frequency domain
allocated to the user.

[0175] The frequency offset estimation unit 26A estimates a frequency
offset amount, using the channel estimation value obtained, and the
maximum path window information obtained from the received PRACH signal
(details of which will be described later).

[0176] Next, the obtained channel estimation values and a data signal
after the subcarrier demapping are supplied to the data equalizer 27.
which performs a frequency domain equalization on the data signal.

[0178] The following describes details of the maximum path detection units
16R, 16C and 16L and the maximum path window selection unit for the PRACH
in the exemplary embodiment of the present invention.

[0181] Next, each of the maximum path detection units 16R, 16C, and 16L
for the center window, the right window, and the left window searches the
maximum path, as shown in the following expressions (20), (21), and (22)
to obtain power of the maximum path for each window. The center, right,
and left windows are ranges respectively defined as Wcenter,
Wright, and Wleft in expressions (20), (21), and (22).

[0185] In the above expressions (20), (21), and (22), Nsearch is a
path search width, and is generally given by the following expression
(23). It is noted that the path search width is not always limited to
that given by the following expression (23).

[0192] Maximum path power output values Pmax, center, Pmax,
right, Pmax, left from the center, right, and left windows obtained
by the above-mentioned method are supplied to the maximum path window
selection unit 17 to select a maximum path window MaxWindow (=center,
right, or left) according to the following criterion.

[0193] The selection criterion for the maximum path window is given by the
following expression (25), for example. Information on the selected
maximum path is supplied to the frequency offset estimation unit 26A for
the PUSCH.

[0194] Next, the channel estimation unit 25 obtains, for each subcarrier
allocated to the user, a complex channel estimation value:

H(s,k) (26)

from the received PUSCH signal.

[0195] where

[0196] (slot number in one subframe)=0, 1.

[0197] k (subcarrier number)=0, 1, . . . , N-1; N is the number of
subcarriers allocated to the user). The information on the maximum path
window obtained from the received PRACH signal of the user by the
above-mentioned method is supplied to the frequency offset estimation
unit 26A of the PUSCH receiver, together with data on complex channel
estimation values.

[0198] The frequency offset estimation unit 26A calculates the complex
correlation value R indicated by the above expression (3). The calculated
correlation value R and the information on the maximum path window
(=center, right, or left) obtained from the received PRACH signal are
combined to estimate a frequency offset based on the criterion indicated
by one of the following expressions (27) to (30).

[0199] The frequency offset estimation value Δf[Hz] calculated by
the above-mentioned technique is supplied to the demodulation unit 28 of
the PUSCH receiver and the demodulation unit 28 perform frequency offset
compensation. That is, the demodulation unit 28, performs the frequency
offset compensation on the signal converted to the time domain signal,
using the frequency offset amount estimated by the frequency offset
estimation unit 26A, as in the related art. The frequency offset
compensation performed on the signal converted into the time domain
signal is given by the above-mentioned expression (1), for example.

[0201] In the above description, for simplicity, discussion was made,
without taking into consideration an influence of an interference (noise)
component. In actual reception processing, some interference component is
certainly added to a received signal, so that it is necessary to take the
influence of the interference component into consideration. Then, error
events E0, E1, E2, and E3 listed below will be considered in order to
examine the influence of the interference (noise) component on the
present invention. Each of the events (E0 to E3) listed below indicates
an event in which an estimated frequency offset greatly deviates due to a
selection error of maximum window information calculated from a received
PRACH signal caused by an interference component included in a complex
correlation value.

E0: event in which MaxWindow≠Right under a condition:
Δf≧1000 [Hz] E1: event in which MaxWindow=Left under a
condition: 0[Hz]≦Δf≦1000 [Hz] E2: event in which
MaxWindow=Right under a condition: -1000 [Hz]≦Δf≦0
[Hz] E3: event in which MaxWindow≠Left under a condition:
Δf≦-1000 [Hz]

[0202] Assuming that probabilities at which the events E0, E1, E2, and E3
will occur are respectively defined to be Pr0, Pr1, Pr2, and Pr3, Pr3=Pr0
and Pr2=pr1 hold due to symmetry. Thus, attention will be hereinafter
focused on Pr0 and Pr1 alone.

FIG. 19 shows the error probabilities Pr0 and Pr1 with respect to a
reception S/N [dB] (decibel representation of a signal to noise power
ratio). Referring to FIG. 19, triangles connected by a solid line (thin
line) indicates Pr0 (S=1), circles connected by a solid line (thin line)
indicates Pr1 (S=1), triangles connected by a broken line (thick line)
indicates Pr0 (S=1.2), and circles connected by a broken line (thick
line) indicates Pr1 (S=1.2).

[0203] As shown in FIG. 19, in case there is no scaling S (equivalent to
S=1), frequencies of occurrence of the two different error events E0 and
E1 (Pr0 (S=1) and Pr1 (S=1) indicated by the solid lines) greatly differ,
so that one of the error events (=E1) becomes dominant. That is, in this
case, the probability at which the maximum window (MaxWindow) for the
frequency offset in the range of -1000 Hz to 1000 Hz is erroneously
selected becomes greater than the probability at which the maximum window
(MaxWindow) for the frequency offset in the range of 1000 Hz or more or
in the range of -1000 Hz or less is erroneously selected.

[0204] On contrast therewith, in case S is set to be 1.2, frequencies of
occurrence of the two events become close as shown in FIG. 19 (by the
broken lines Pr0 (S=1.2) and Pr1 (S=1.2)), so that the two error events
are balanced within the ranges of all the frequency offsets to be
compensated for. In other words, the probabilities of occurrence of
selection errors of the maximum windows (MaxWindows) for any users can be
made uniform, irrespective of the magnitude of a frequency offset amount
for each user.

[0205] Processing of each of the maximum path detection units 16C, 16L,
and 16R, that detects the maximum path based on a signal (digital signal)
from the IDFT unit 15, processing of the maximum path window selection
unit 17 that selects the maximum path window in the PRACH reception
processing unit in FIG. 17 and frequency offset estimation processing of
the frequency offset estimation unit 26A in the PUSCH reception
processing unit in FIG. 18 may be of course implemented by a program
(software processing) to be executed on a computer. According to the
present invention, there is provided a storage medium (a memory device, a
magnetic/optical disk medium/device) having the program stored therein.

[0206] As described above, according to this exemplary embodiment,
operation and effect as will be described below are achieved.

[0207] By combining maximum window information obtained from a received
PRACH signal, in addition to a correlation value between channel
estimation values obtained from a received PUSCH signal, in order to
estimate a frequency offset, it is made possible to expand an estimable
frequency offset range from

- 1 2 1 T S [ Hz ] ~ 1 2 1 T S [ Hz ]
##EQU00021##

to

- 1 T S [ Hz ] ~ 1 T S [ Hz ]
##EQU00022##

where Ts is a duration between two PUSCH reference symbols in one
sub frame.

[0208] Further, by introducing the scaling value (S) as the selection
criterion in the maximum window selection unit, it is made possible to
make the probabilities of occurrence of selection errors of the maximum
windows (MaxWindows) for all users uniform, irrespective of the magnitude
of a frequency offset amount for each user.

[0209] Modifications and adjustments of the exemplary embodiments and an
example are possible within the scope of the overall disclosure
(including claims) of the present invention, and based on the basic
technical concept of the invention. Various combinations or selections of
various disclosed elements are possible within the scope of the overall
disclosure of the present invention. That is, the present invention of
course includes various variations and modifications that could be made
by those skilled in the art according to the overall disclosure including
the claims and the technical concept.